Scintillation detector array for encoding the energy, position and time coordinates of gamma ray interactions

ABSTRACT

A scintillation detector which includes a plurality of discrete scintillators composed of one or more scintillator materials. The discrete scintillators interact with incident radiation to produce a quantifiable number of photons with characteristic emission wavelength and decay time. A light guide is operatively associated with the scintillation crystals and may be either active or non-active and segmented or non-segmented depending upon the embodiment of the design. Photodetectors are provided to sense and quantify the scintillation light emissions. The process and system embodying various features of the present invention can be utilized in various applications such as SPECT, PET imaging and simultaneous PET systems. In accordance with the present invention, the detector array of the present invention incorporates either a single scintillator layer of discrete scintillators or discrete scintillators composed of two stacked different layers that can be the same scintillator material or of two different scintillator materials. In either case the different layers are composed of materials that have distinctly different decay times. The variants in these figures are the types of optical detectors which are used, i.e. photomultipliers and/or photodiodes, whether or not a segmented optical planar light guide is used, and whether the planar light guide is active or non-active. If a segmented optical planar light guide is used then the variant is whether the configuration is inverted or non-inverted.

[0001] This is a continuation of application Ser. No. 09/882,101, filedon Jun. 15, 2001, which was a continuation-in-part of application Ser.No. 09/271,770, filed on Mar. 18, 1999, which was a non-provisionalapplication that claimed the priority benefit of U.S. ProvisionalApplication No. 60/079,279, filed Mar. 25, 1998. Accordingly, thisnon-provisional application also claims the priority benefit of U.S.Provisional Application No. 60/079,279, filed Mar. 25, 1998.

TECHNICAL FIELD

[0002] The present invention relates to an apparatus capable ofdetermining the energy, position and time coordinates of light emissioninduced by interactions of gamma-rays in a planar array of discretescintillator detectors having either a segmented or non-segmented lightguide. The features of the present invention find particular applicationin the field of medical imaging whereby a single device can be used forSingle Photon Imaging which includes traditional Gamma Cameras, PlanarImaging, Single Photon Emission Computed Tomography (SPECT) with orwithout Coincidence Photon Imaging and Positron Emission Tomography(PET). When operated in the SPECT mode, the present invention iscomparable to existing high resolution SPECT systems. When operated inthe PET mode, the present invention is an improvement over existing PETsystems in that the device may be operated either in Pulse HeightDiscrimination mode or in Pulse Shape Discrimination mode therebyenabling depth of interaction encoding resulting in improved spatialresolution. Emission Computed Tomography (ECT) systems provide a meansfor sensing, and quantitatively measuring biochemical and/orphysiological changes in the human body or other living organism.However, the use of the invention is not limited to such application.

BACKGROUND ART

[0003] Devices for detecting the distribution of gamma rays transmittedor emitted through objects to study the compositions or functions of theobjects are well known to the art, e.g. the techniques referred to asEmission Computed Tomography can be divided into two specific classes;Single Photon Emission Computed Tomography (SPECT) uses radiotracerswhich emit gamma rays but do not emit positrons and Positron EmissionTomography (PET) which uses radiotracers that emit positrons. Therefore,the fundamental physical difference between the two techniques is thatPET uses annihilation coincidence detection. The PET technique candetermine, in-vivo, biochemical functions, on the injection ofbiochemical analog radiotracer molecules that emit positrons in a livingbody. The positrons annihilate with surrounding electrons in the subjectbody to produce a pair of gamma-rays, each having 511 keV of photonenergy; traveling in nearly opposite directions. The detection of a pairof annihilation gamma-rays by two opposed detectors allows for thedetermination of the location and direction in space of a trajectoryline defined by the opposite trajectories of the gamma-rays. Tomographicreconstruction is then used to superpose the numerous trajectory linesobtained by surveying the subject with an array of detectors to imagethe distribution of radiotracer molecules in the living body.

[0004] Emission Computed Tomography systems employ a variety ofgeometric configurations for the gamma-ray detectors. The choice ofconfiguration is typically dictated by the manufacturer's desired systemperformance and cost. The detector design must be capable of providingaccurate estimates of gamma-ray energy, position coordinates, and inaddition in the case of PET, coincidence time interval to reconstruct animage of the distribution of the radiotracer for in vivo studies. Anexample of such a device is disclosed in U.S. Pat. No. 4,750,972 toCasey et al., the disclosure of which is incorporated herein byreference and relied upon.

[0005] The position encoder and detector system disclosed by Casey et.al., is a two dimensional photon counting position encoder detectorsystem, i.e., the array of scintillation crystals provides only thetransverse coordinates of the photon interaction; the longitudinalphoton interaction position of the excited scintillation crystal isundetermined. Photons impinging upon such detector systems at anglesother than normal may traverse the path of several scintillationcrystals resulting in uncertainty of their trajectory lines therebydegrading the image resolution due to parallax error.

[0006] In U.S. Pat. No. 3,919,556 by Berninger, Berninger discloses agamma camera having a light pipe member in which the output member has aplurality of concave depressions conforming to the outer surfaces of theconvexly curved phototube glass faceplates. Berninger teaches that theprimary function of the light pipe is simply that of providing arefractive index match between the glass backing of the scintillator andthe glass envelope of the phototubes.

[0007] A detector system capable of providing both the transverse andlongitudinal position of photon interactions in scintillation crystalswas disclosed in U.S. Pat. No. 4,843,245 by Lecomte. The approachinvolves the use of two scintillation crystals of different decay timeswhich are stacked one upon the other. The position of photon interactionis determined by the Pulse Shape Discrimination technique. This methodthough capable of providing the transverse and longitudinal positioncoordinates of photon interactions in scintillation crystal detectorsystems will result in reduced system efficiency if the overallscintillator depth is constant for two different scintillator materials.If the scintillators are increased in length to compensate for theefficiency loss then the system resolution will be degraded.

[0008] Another approach to determine the transverse and longitudinalpositions of photon interactions in scintillation crystal detectorsystems was disclosed in U.S. Pat. No. 5,122,667 by Thompson. Theapproach differs from that of Lecomte in that a single scintillator isused, further the method does not depend on decay time differences. Themethod employs the use of a scintillation light absorbing band locatedat the median interaction coordinate for a specific energy along thelongitudinal axis of the scintillation crystal. The net effect is todivide the scintillation crystal into two regions whereby the photon isequally likely to interact. Pulse Height Discrimination is used todetermine which of the two regions of the scintillator the photoninteracted. This approach has the undesired effect of reducing the totalcollected scintillation light and of causing the Compton continuum ofthe high light yield scintillator to overlap the photopeak region of thelow light yield scintillator. The result is inherent uncertainty in thecontribution of scatter to the full energy photopeak.

[0009] In U.S. Pat. No. 5,349,191 Rogers discloses a method fordetermining the transverse and longitudinal position coordinates forinteractions in scintillation crystal arrays which depends on thecontinuous variation of the total collected light with the longitudinalphoton interaction coordinate of the light emission. The continuousvariation in collected light requires a complex calibration of eachdetector as a function of longitudinal photon interaction coordinatefrom a collimated beam of photons directed at known positions along thelength of the scintillator. This calibration method is difficult toimplement for large arrays of scintillators.

[0010] In U.S. Provisional Application Serial No. 60/037,519, filed onFeb. 10, 1997, and U.S. Provisional Application Serial No. 60/042,002,filed on Apr. 16,1997, Moisan and Andreaco et. al. disclosed a devicecapable of determining the transverse and longitudinal coordinates oflight emission induced by the interaction of photons in an array ofphoton detectors having a plurality of scintillation light guides. Thedevice uses two or more layers of stacked scintillators all composed ofthe same scintillator material. Pulse Height Discrimination is used todetermine which scintillator layer the photon interaction occurs. Thedevice requires a difference in the light output from the two stackedscintillator layers of at least a factor of 1.5 times for the pulseheight discrimination technique to be practicable. The approach has theundesired effect of causing the Compton continuum of the high lightyield scintillator (which is nearest to the subject under study) tooverlap the photopeak region of the low light yield scintillator. Theresult is inherent uncertainty in the contribution of scatter to thefull energy photopeak.

[0011] The detector systems described in the above stated US Patentswhen applied to medical imaging are specific to usage in PET. Thepredominant scintillator material is Bismuth Germanate (BGO), thoughother materials have been proposed or used (see Table 1). The SPECTdetector systems are different in that Thallium doped Sodium-Iodide(NaI(Tl)) is used exclusively as the scintillator material. Furtherthese systems use large continuous slabs of NaI(Tl) optically coupled toa continuous light guide. Anger logic is used for scintillation eventlocalization. The exception to continuous NaI(Tl) slab detector systemsfor SPECT imaging was disclosed by Govaert in U.S. Pat. No. 4,267,452.This detector system is unique as a SPECT detector in that it issegmented. The segmentation of the NaI(Tl) is similar to PET blockdetector designs which use an active light guide. (For clarificationdetector light guides are of two general types: non-active light guidesare composed of optical materials other than the scintillator; activelight guides are composed of scintillator materials). The detectorsystem disclosed by Govaert does not result in discrete scintillatorelements whereby each element is a separate detector. Instead thesegmentation process results in a block of NaI(Tl) that is subdividedinto elements that share a common light guide of active scintillatormaterial, i.e. the NaI(Tl) is not cut all the way through.

[0012] The unique differences in SPECT and PET imaging modalities haveresulted in detector designs which are suitable for their intended usein either SPECT or PET, but not both. However, the use ofFluorodeoxyglucose (FDG) with SPECT imaging systems has resulted in theapplication of SPECT detector designs in PET imaging. One problem in theapplication of SPECT detector designs in PET is that relatively thinscintillation crystals are preferred in Anger cameras to provide betterintrinsic resolution and image detail. This results in poor detectionefficiency in PET since the effective-Z and density of NaI(Tl) provideslower stopping power at 511 keV relative to PET scintillators (see Table1). The efficiency of SPECT detector systems is further reduced by theuse of absorptive collimation. The continuous slab of NaI(Tl) precludesthe elimination of absorptive collimation.

[0013] SPECT detector system designs which are intended to bridge bothSPECT and PET imaging modalities are known as hybrid devices. Thesesystems have increased the NaI(Tl) scintillator thickness for higherefficiency and have added coincidence detection circuitry andattenuation corrections. Despite these changes the continuous slab ofNaI(Tl) scintillator detector designs are inferior to PET specificdetector designs in terms of system performance.

[0014] The hybrid SPECT detector designs have compromised their SPECTperformance while providing inferior PET performance. A need has arisenfor a hybrid PET/SPECT detector system which provides state of the artSPECT and PET system performance which does not suffer from theheretofore stated disadvantages.

DISCLOSURE OF THE INVENTION

[0015] In accordance with the various features of this invention, ascintillation detector is provided which includes a plurality ofdiscrete scintillators composed of one or more scintillator materials.The discrete scintillators interact with incident radiation to produce aquantifiable number of photons with characteristic emission wavelengthand decay time. A light guide is operatively associated with thescintillation crystals and may be either active or non-active andsegmented or non-segmented depending upon the embodiment of the design.Photodetectors are provided to sense and quantify the scintillationlight emissions. The process and system embodying various features ofthe present invention can be utilized in various applications such asSPECT and PET imaging systems. In accordance with the present invention,the detector array of the present invention incorporates either a singlelayer of discrete scintillators or discrete scintillators composed oftwo stacked different layers that can be the same scintillator materialor of two different scintillator materials. In either case the differentlayers are composed of materials that have distinctly different decaytimes. The variants in these figures are the types of optical detectorswhich are used, i.e. photomultipliers and/or photodiodes, whether or nota segmented optical light guide is used, and whether the light guide isactive or non-active. If a segmented optical light guide is used thenthe variant is whether the configuration is inverted or non-inverted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above mentioned features of the invention will become moreapparent from consideration of the following description when readtogether with the accompanying drawings, in which:

[0017]FIG. 1 is a perspective view of a medical imaging scannerembodying scintillation detector arrays for encoding the energy,position and time coordinates of gamma-ray interactions.

[0018]FIG. 2 is comprised of FIG. 2a through and inclusive of FIG. 2e.These figures present various views of the detector head of the systemillustrated in FIG. 1 and the detector blocks as mounted in the detectorhead. FIG. 2a depicts an 8×10 array of detector blocks optically coupledto a 9×11 array of photomultiplier tubes. FIGS. 2b and 2 c illustratecross-sectional views of the detector head in which the array ofdetector blocks are optically coupled to an array of photomultipliertubes. FIGS. 2d and 2 e illustrate cross-sectional views of the detectorhead in which the array of detector blocks is optically coupled to aphotodiode array.

[0019]FIG. 3 is comprised of FIG. 3(a) through and inclusive of FIG.3(l). These figures present a perspective view of the types of detectorblocks which could be incorporated in the detector head illustrated inFIG. 2. The common feature in these figures is the embodiment of thedesign incorporating discrete scintillators composed of two stackeddifferent scintillator materials of different decay times. The variantsin these figures are the types of optical detectors which are used, i.e.photomultipliers and/or photodiodes. The other variant is whether or nota segmented optical light guide is used. If a segmented optical lightguide is used then one variant is whether the configuration is invertedor non-inverted. Another variant is whether the light guide is active ornon-active.

[0020]FIG. 3(a) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for one preferred embodiment of thedesign incorporating discrete scintillators composed of two stackeddifferent scintillator materials of different decay times opticallycoupled to a non-active segmented inverted light guide which isoptically coupled to photomultiplier tubes (PMTs). The PMTs are theoptical detectors.

[0021]FIG. 3(b) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of two stacked differentscintillator materials of different decay times optically coupled to anon-active segmented inverted light guide. The light guide is opticallycoupled to PMTs in addition a photodiode array is optically coupled toone of the scintillator arrays. The PMTs and photodiode arrays are theoptical detectors.

[0022]FIG. 3(c) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of two stacked differentscintillator materials of different decay times optically coupled to anon-active segmented inverted light guide. The light guide is opticallycoupled to a photodiode array. The photodiode array is the opticaldetector.

[0023]FIG. 3(d) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of two stacked differentscintillator materials of different decay times optically coupled to anon-active segmented inverted light guide. The light guide is opticallycoupled to a photodiode array in addition a second photodiode array isoptically coupled to one of the scintillator arrays. The two photodiodearrays are the optical detectors.

[0024]FIG. 3(e) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of two stacked differentscintillator materials of different decay times. One of the twoscintillator arrays is optically coupled to a photodiode array. Thephotodiode array is the optical detector.

[0025]FIG. 3(f) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of two stacked differentscintillator materials of different decay times. One photodiode array isoptically coupled to one of the scintillator arrays. A second photodiodearray is optically coupled to the other scintillator array. Thephotodiode arrays are the optical detectors.

[0026]FIG. 3(g) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of two stacked differentscintillator materials of different decay times optically coupled to anon-active segmented non-inverted light guide. The light guide isoptically coupled to PMTs. The PMTs are the optical detectors.

[0027]FIG. 3(h) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of two stacked differentscintillator materials of different decay times optically coupled to anon-active segmented non-inverted light guide. The light guide isoptically coupled to the PMTs in addition a photodiode array isoptically coupled to one of the scintillator arrays. The PMTs and thephotodiode arrays are the optical detectors.

[0028]FIG. 3(i) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of two stacked differentscintillator materials of different decay times optically coupled to anon-active segmented non-inverted light guide. The light guide isoptically coupled to a photodiode array. The photodiode array is theoptical detector.

[0029]FIG. 3(j) is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of two stacked differentscintillator materials of different decay times optically coupled to anon-active segmented non-inverted light guide. The light guide isoptically coupled to a photodiode array. A second photodiode array isoptically coupled to one of the scintillator arrays. The photodiodearrays are the optical detectors.

[0030]FIG. 3(k) is a perspective view of an alternative detector blockapplicable to any of the embodiments discussed herein showing that thelight depth and configuration of the segmentation is variable dependingupon the thickness of the light guide.

[0031]FIG. 3(l) is a perspective view of an alternative detector blockapplicable to any of the embodiments discussed herein showing that theperimetric dimensions of a single detector block, which in oneembodiment is defined by a 12×12 discrete element array can becoextensive with the perimetric dimension of a 2×2 array of four opticaldetectors. The segmented light guide can be inverted or non-inverted,active or non-active.

[0032]FIG. 4 is inclusive of FIGS. 4, 4a and 4 b. FIG. 4 and 4 aillustrate a side elevation and plan, respectively, of one detectorblock from the detector head illustrated in FIG. 2 for anotherembodiment of the design incorporating discrete scintillators composedof two stacked different scintillator materials of different decay timesoptically coupled to a non-active non-segmented planar light guide. Theplanar light guide is optically coupled to the PMTs. The PMTs are theoptical detectors. FIG. 4b illustrates an alternate embodiment in whichthe discrete scintillators are disposed between the light guide and theoptical detectors.

[0033]FIG. 5 is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of a single scintillatormaterial optically coupled to a non-active segmented light guide. Thelight guide may be inverted or non-inverted. The light guide isoptically coupled to the PMTs. The PMTs are the optical detectors.

[0034]FIG. 6 is inclusive of FIGS. 6, 6a and 6 b. FIGS. 6 and 6aillustrate side elevation and plan views, respectively, of one detectorblock from the detector head illustrated in FIG. 2 for anotherembodiment of the design incorporating discrete scintillators composedof a single scintillator material optically coupled to a non-activenon-segmented planar light guide. The planar light guide is opticallycoupled to the PMTs. The PMTs are the optical detectors. FIG. 6billustrates an alternate embodiment in which the discrete scintillatorsare disposed between the light guide and the optical detectors.

[0035]FIG. 7 is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of a single scintillatormaterial of two different decays times optically coupled to a non-activesegmented inverted light guide. The light guide is optically coupled tothe PMTs. The PMTs are the optical detectors.

[0036]FIG. 8 is a perspective view of one detector block from thedetector head illustrated in FIG. 2 for another embodiment of the designincorporating discrete scintillators composed of a single scintillatormaterial of two different decay times optically coupled to a non-activenon-segmented planar light guide. The light guide is optically coupledto the PMTs. The PMTs are the optical detectors.

[0037]FIG. 9(a) depicts the uniform irradiation by a Na-22 radioactivesource at 511 keV of a single layer of NaI(Tl) segmented into a 12×12array yielding 144 discrete elements. The detector design is exhibitedin FIG. 5. The position histogram displayed in FIG. 9(a) is of theoverlap of twelve rows each containing 12 discrete scintillatorelements. The figure displays excellent peak-to-valleys indicating verygood separation among the 144 discrete elements.

[0038]FIG. 9(b) is the same as FIG. 9(a) except Co-57 is used as theradioactive source with a gamma-ray energy of 122 keV.

[0039]FIG. 10 is the same as FIG. 9(a) except Cd-109 is used as theradioactive source with a gamma-ray energy of 88 keV.

[0040]FIG. 11 is the same as FIG. 9(b) except the peak-to-valleys arenot displayed. The position histogram displays all 144 discrete NaI(Tl)scintillator elements for the detector design as exhibited in FIG. 5when uniformly irradiated with 122 keV gamma-rays from Co-57.

[0041]FIG. 12 is the position histogram displaying all 144 discreteNaI(Tl) scintillator elements for the detector design as exhibited inFIG. 3(a). In this case one scintillator layer (slow) is composed ofNaI(Tl) and the other scintillator layer (fast) is composed of LSO. Thedetector is uniformly irradiated with 140 keV gamma-rays from Tc-99m.

[0042]FIG. 13 is the Cross-over time spectra and position histograms forthe detector design as exhibited in FIG. 3(a). In this case onescintillator layer (slow) is composed of NaI(Tl) and the otherscintillator layer (fast) is composed of LSO. The detector is uniformlyirradiated by 511 keV gamma-rays from Ge-68. Pulse Shape Discriminationas provided in FIG. 18 is used to determine in which of the twoscintillator layers the gamma-rays interacted. The cross-over timespectra is shown in FIG. 13 for the two scintillator layers. Theposition histograms are labeled ‘slow’ for the NaI(Tl) layer and ‘fast’for the LSO layer. The position histograms exhibit excellent separationof the 144 discrete elements for each scintillator layer. FIG. 13illustrates the effectiveness of the detector design and the pulse shapeseparation technique.

[0043]FIG. 14 is a pulse height energy spectrum for an LSO crystalirradiated by 662 keV gamma-rays from Cs-137. Also exhibited in thespectrum is the 2.6% abundant Lu-176 background of LSO.

[0044]FIG. 15 exhibits the energy integration for NaI(Tl) and LSO. Thepulse shape discrimination technique utilized with the detector designsof this disclosure, involves integrating the detector charge at twopoints in time then ratio the values. For LSO and NaI(Tl) separation thefirst sample is taken at 80 ns from the start of the integration and thesecond sample at 256 ns at the end of signal integration to provide avalue to normalize out event charge (energy).

[0045]FIG. 16 In the two-sample shape discrimination, the integratedenergy signal is sampled using an 8-bit flash converter at approximately80 ns (E1) and again at 256 ns (E2). The energies (E1) and (E2) are usedto determine if the event occurred in a NaI(Tl) or an LSO crystal. Theratios of the energies (E1) and (E2) are used for the shapediscrimination. A 65536×1 static RAM can be used to indicate the crystaltype as shown in FIG. 16.

[0046]FIG. 17 The integrated X and Y values are digitized at 256 nsusing flash converters with the integrated energy signal as thereference to produce the (A+B)/Sum and (A+C)/Sum ratios. The X and Yratios are used to determine the crystal in which the event occurred. A65536×8 static RAM can be used to indicate the crystal as shown in FIG.17.

[0047]FIG. 18 displays the block diagram of the setup used to evaluatethe detectors.

[0048]FIG. 19 displays the block diagram for the coincidence setup.

[0049]FIG. 20 displays the geometric arrangement used in coincidencetiming measurements.

[0050]FIG. 21 displays the coincidence time spectra for the variousscintillator layer combinations.

[0051]FIG. 22 displays the contour plots for coincidence time centroidsin nsec for the various scintillator layer combinations. The zero-lineis based on the mean LSO-LSO centroid position. The time centroids aresymmetric, but the centroid position is dependent upon the location ofthe discrete scintillator element with respect to the PMT and reflectsthe spatial uniformity in the anode output. The time centroid shifts canbe corrected via a lookup table.

[0052]FIG. 23 similarly displays the surface plots for coincidence timecentroids in nsec for the various scintillator layer combinations.

[0053]FIG. 24 displays the geometric arrangement used in the line spreadfunction measurements.

[0054]FIG. 25 displays the line spread functions for the variousscintillator layer combinations.

[0055]FIG. 26 displays the general system architecture of the medicalimaging system.

BEST MODE FOR CARRYING OUT THE INVENTION

[0056] The present invention is a scintillation detector array forencoding energy, position and time coordinates of gamma ray interactionsfor use in Single Photon Emission Computed Tomography, (“SPECT”), withor without coincidence photon imaging, Planar Imaging, and PositronEmission Tomography, (“PET”), imaging. Referring now to the drawings,FIG. 1 depicts a perspective view of a medical imaging scanner 12embodying scintillation detector arrays 10 for encoding the energy,position and time coordinates of gamma-ray interactions. The detectorhead assembly 15 comprises an (n)×(m) planar array of detector blocks20, optically coupled to an (q)×(p) array of photomultiplier tubes(PMTs) 25 in one embodiment of the design; or to an (y)×(z) array ofoptical detectors such as Avalanche Photodiodes (APDs) or PINPhotodiodes 25′ in another embodiment of the design. Note the variables(n),(m),(q),(p),(y),(z), may or may not equal each other. FIG. 2adepicts an 8×10 planar array of detector blocks 20 optically coupled toa 9×11 planar array of photomultiplier tubes 25 configured so that thecenter of each PMT 25 resides over the corner of each detector block 20.While this is the preferred arrangement, where the variables(n),(m),(q),(p),(y),(z) are equal the scintillator elements and theoptical detectors can be positioned so as to be co-linear. As seen inFIGS. 2b and 2 c, the PMTs 25 are partitioned into 99 squarecompartments using 0.25 mm thick magnetic shielding arranged in a crosspattern to form a grid that locates the PMTs 25 and provides structuralsupport for the 1.0 mm glass window 30 that separates the PMTs 25 fromthe detector blocks 20. An hermetic enclosure is provided by a thinstainless steel foil 35 membrane mounted to the subject side of theenclosure. As seen in FIGS. 2d and 2 e, a similar arrangement isutilized when the optical detector is a photodiode array.

[0057]FIG. 3(a) is a perspective view of a detector block 20 from thedetector head 15 of FIG. 2 for one preferred embodiment of the designincorporating discrete scintillators 40 composed of two stackeddifferent scintillator materials, i.e. a slow scintillator material 140and a fast scintillator material 240, respectively, of different decaytimes optically coupled to a segmented inverted light guide. Thoseskilled in the art will appreciate that the light guides discussedherein can be either “active” or “non-active”. The term “non-active” isused when the light guide is composed of a non-scintillating material,whereas an active light guide is composed of scintillating material. Theuse of an active light guide has the inherent characteristic ofmispositioning of events which occur as a result of interactions in thenon-segmented portion of the light guide. Provided the active lightguide is properly designed the magnitude of mispositioned events can beminimized, but never completely eliminated. In the preferred embodiment,light guide 50 is non-active.

[0058] Moreover, the term “segmented”, as used herein, describes aplurality of barriers defining a preselected number of slots, the numberof which and the depth of which are varied to control the variablestatistical distributions of photons, whereas non-segmented light guidesare continuous such as those used in conventional gamma cameras. And, asused herein, the term “inverted” is used to describe a light guide inwhich the slotted section of the light guide is optically coupled to theoptical detectors, whereas for a traditional (non-inverted) light guidethe section of the light guide that contains the non-slotted continuousregion is optically coupled to the optical detectors.

[0059] The selection of type and orientation of the light guide is inresponse to various manufacturing constraints. For example when thescintillator and light guide are composed of different materials theymay have to be processed separately each using a unique set of toolingand chemical processing. Whereas when the scintillator and light guideare composed of the same material then the tooling and chemicalprocessing are generally the same and no bonding agents are required tooptically bond the scintillator to the light guide, since under thiscircumstance the light guide is cut into the scintillator. However, thedetector designer may choose to put an optical bond between thescintillator and the light guide even though they are composed of thesame material for the purpose of depth of interaction encoding by eitherpulse shape or pulse height discrimination. One reason why the detectordesigner may not want to use the scintillator as an active light guideis due to the mispositioning of events which occur as a result ofinteractions in the non-segmented portion of the light guide.

[0060] For an (n)×(m) array of discrete scintillators 40 opticallycoupled to a segmented light guide, such as light guide 50, the cutdepths for an inverted and non-inverted light guide are unique and arenot interchangeable. A detector design incorporating discretescintillators 40 and a non-inverted light guide, such as non-invertedlight guide 150 in the Figures, cannot be converted to a functionalinverted light guide detector of equivalent performance simply byflipping the light guide.

[0061] For the preferred embodiment of the design, an inverted segmentedlight guide 50, which is non-active, is used as depicted in FIG.3(a).The composition of the inverted segmented light guide 50 can be of anymaterial that is chemically compatible with the scintillator materialand is optically transmissive to the wavelength of emission of thescintillator. Other material properties constraints the detectordesigner must consider in selection of the light guide material is theindex of refraction, thermal characteristics, mechanical characteristicsand cost. As stated above, in the preferred embodiment, the invertedsegmented light guide 50 was designed to be non-active so thatmispositioning of events due to gamma-ray interactions in the lightguide would not occur. The light guide was designed to be inverted sothat registrational tolerances could be eased with respect to thecorrespondence of the discrete scintillator elements 40 relative to thepartitioned section of the light guide. The reflector may be of anymaterial that has high reflectance for the emission wavelength of thescintillator(s). In the case of NaI(Tl) and LSO, 1.53 micron Silicondioxide (SiO₂) was selected as the reflector. Other particle diameterscould be used with the general trend as the particle diameter increasesthe optical cross-talk among the discrete scintillator elements as wellas among the partitions of the light guide increases thereby degradingthe signal-to-noise ratio.

[0062] Two different scintillators 140 and 240 having different decaytimes are used in one of the preferred embodiments of the design asillustrated in FIG. 3(a). NaI(Tl) was selected as the scintillator ofchoice for the SPECT measurements and is the current industry standard.The NaI(Tl) block size is 52 mm×52 mm×10 mm thickness. Block sizes ofdifferent cross sections or thickness can be selected by the detectordesigner; the cross-section will be set by the dimensions of the opticaldetector(s) and the thickness based on the level of compromise betweenefficiency gain and resolution degradation. Prior to sawing the NaI(Tl)into segments it must be bonded to a substrate for mechanical integrityduring processing and to preserve the discrete elements positionalregistration. Selection of the bonding agent and substrate requiresconsideration of mechanical, chemical, and optical properties. Thebonding agent and substrate must be optically transmissive to theemission wavelength of the NaI(Tl) scintillator, they must be chemicallycompatible and provide mechanical strength without thermal expansiondetriment. An optical glass slide of 0.5 mm thickness was selected asthe bonding substrate. Other glass slide thicknesses could be selectedbased on the level of compromise among mechanical strength, opticalcross-talk, and optical attenuation. The bonding agent may be eitherepoxy or RTV. Selection of the ‘sawing’ method for segmentation mustconsider thermal and mechanical stresses as well as chemicalcompatibility. NaI(Tl) processing should be performed in a dry room toprevent hydration of the scintillator. The cutting lubricant temperaturemust be controlled to prevent thermal fracture of the scintillator;further it must be chemically compatible so that the mechanicaldimensions and optical properties of the scintillator do not change withtime. Regardless of which cutting method or lubricant is selected allresidues from the process must be removed from the scintillator surfaceotherwise the total light emitted will be reduced. A low viscosity oilsimilar in viscosity to water was selected as the cutting lubricant.Residues from the cutting process were subsequently removed eitherchemically or mechanically.

[0063] The 52 mm×52 mm×10 mm NaI(Tl) block was bonded to a 0.5 mm thickglass substrate and sawed into a 12×12 discrete element array of 4.37 mmpitch and 4.0 mm×4.0 mm×10.0 mm crystal size. Silicon dioxide powder of1.53 micron particle size is used as the reflector in the interstices 55of the 144 elements of the array and is also used as the reflectorbetween the blocks and on the scintillator non-interstitial surfaces.The silicon dioxide reflector in the interstices 55 is ‘sealed’ with asmall layer of Teflon powder. This is to prevent the permeation ofbonding agents into the silicon dioxide reflector from subsequentbonding processes.

[0064] Referring to FIG. 3(a). LSO was selected as the otherscintillator of the pair due to its high luminosity, high density,effective-Z and its fast decay time (see Table 1). TABLE 1 Properties ofProposed PET Scintillators NaI(Tl) BGO LSO LOP GSO CeF₃ Density (gm/cm³)3.67 7.13 7.4 6.53 6.71 6.16 Effective Z 50.6 74.2 65.5 62.5 58.6 52.7Mean Free Path (cm)* 2.93 1.05 1.16 1.37 1.43 1.71 Index of Refraction1.85 2.15 1.82 1.7 1.91 1.68 Hydroscopic? YES NO NO NO NO NO Rugged? NOYES YES YES NO Decay Time (ns) 230 300 40 24 60 27 Emission Peak (nm)410 480 420 360 440 340 LightOutput[NaI(Tl) = 100] 100 15 75 32 25 4-5Energy Resolution* 7.8 10.1 10.1 8.9 20 Photoelectric Fraction* .175.411 .324 .288 .247 .188 Incoherent Fraction* .790 .543 .629 .670 .712.778 Mass Attenuation (cm²/gm)* .0930 .1332 .1170 .1118 .1040 .0951 MassEnergy Absorp (cm²/gm)* .0409 .0731 .0601 .0553 .0495 .0424 LinearAttenuation (cm⁻¹)* .3411 .9496 .8658 .7302 .6978 .5858 Linear EnergyAbsorp (cm−1)* .1501 .5214 .4447 .3612 .3323 .2614 BaF₂ PbSO₄ PbCO₃LAP⁽¹⁾ LuAG⁽¹⁾ YSO⁽⁴⁾ Density (gm/cm³) 4.89 6.2 6.6 8.34 6.9 4.543Effective Z 52.2 73.1 75.9 63.9 61.7 34.2 Mean Free Path (cm)* 2.20 1.221.10 1.05 1.31 2.58 Index of Refraction 1.56 1.88 1.8 1.8 Hydroscopic?Slight NO NO NO NO NO Rugged? YES YES YES Decay Time (ns) 0.6 136 8.511⁽²⁾ 58⁽³⁾ 70 Emission Peak (nm) 220 350 475 390 500 420LightOutput[NaI(Tl) = 100] 5 9 1.4 17 25 118 Energy Resolution* 11.4 4042 14.9 8.0 Photoelectric Fraction* .186 .399 .432 .305 .277 .051Incoherent Fraction* .779 .556 .520 .651 .683 .927 Mass Attenuation(cm²/gm)* .0929 .1320 .1382 .1141 .1105 .0853 Mass Energy Absorp(cm²/gm)* .0414 .0718 .0775 .0574 .0540 .0315 Linear Attenuation (cm⁻¹)*.4545 .8181 .9122 .9515 .7623 .3875 Linear Energy Absorp (cm−1)* .2024.4449 .5113 .4790 .3727 .1431

[0065] LSO is a rugged scintillator and its processing methods areunique and different from NaI(Tl). Preferably, the 52 mm×52 mm×10 mm LSOblock is either mechanically polished or etched to transparency prior tobonding or sawing. The reason the mechanical and/or etch process is usedis to maximize the transmission of the other scintillator's lightthrough the LSO and the light guide 50. This mechanical polishing oretching to transparency condition also applies to the light guide. Thoseskilled in the art will recognize that while polishing or etching tooptical transparency is preferred, a polish or etch that results in lessthan optical transparency may alleviate certain manufacturing problemsassociated with etching to optical transparency. However, polishing oretching to less than optical transparency reduces the efficiency of thedetector and/or light guide. Pyrophosphoric acid (H₄P₂O₇) is used as thechemical etchant for LSO. Etching LSO to transparency is a function oftime and acid temperature. Typically a temperature of 300° C. withapproximately 15 minutes duration is required to etch LSO totransparency, other temperatures and durations may also be used to thesame effect. One problem associated with this etching temperature isthermal stress which can fracture the scintillator. Thermal stress canbe minimized by having the scintillator and the acid at the sametemperature during the etch process. Upon removal of the LSO from theetch bath it is allowed to air cool to 100° C. whereupon the LSO issubmerged in boiling water to rinse the residual pyrophosphoric acidfrom the LSO surface. Upon removal of the LSO from the boiling waterrinse bath it is allowed to air dry to room temperature. The LSO is thensubmerged in a 37% Hydrochloric acid (HCl) bath for approximately 2minutes to remove residual pyrophosphoric acid from the LSO which wasnot removed by the boiling water rinse. The duration of the HCl rinse isnot critical as it will not etch the LSO, it will only remove surfacecontaminants. Following the HCl etch a clean water rinse is used toremove residual HCl from the LSO surface.

[0066] The mechanically polished and/or etched uncut LSO block isoptically bonded to a mechanically polished optical grade glass orultraviolet transmissive plastic whose dimensions are 52 mm×52 mm×12.7mm thickness. Other thicknesses can be used depending on the desiredstatistical distribution of photons and optical transmission and opticalattenuation compromise. The light guide 50 can be cut or uncut prior tobonding to the LSO, the choice really depends upon the fabricationprocesses selected. The end result should yield the desired discreteelement registration to light guide partition.

[0067] The 52 mm×52 mm×10 mm LSO block is cut into a 12×12 element arrayof 4.37 mm pitch and 4.0 mm×4.0 mm×10 mm crystal size. The cut LSOcrystal array must be etched to remove surface contaminants due to thesawing process. This etch is generally not designed to be opticallytransparent since the etch at that temperature would be detrimental tothe optical bond. Cleaning the crystal surface can be accomplished withan HCl etch as stated above. However enhancing the LSO light outputafter the sawing process requires an HCl etch to remove the surfacecontaminants from the sawing process followed by a pyrophosphoric acidetch at a nominal temperature of 170° C. for a duration of approximately20-30 seconds. Etching for shorter or longer duration at thistemperature will not provide the optimum light yield. Otherpyrophosphoric acid temperatures and etch times may be used. The choicedepends upon the manufacturing process selected and the amount of timeallocated to the etch process and the desired light output. Thepyrophosphoric acid etch removes the sawing process inducedmicro-fracturing of the LSO crystal surface which would otherwise trapthe LSO luminescence light. The pyrophosphoric acid etch is followed bya 100° C. water rinse then a second HCI rinse followed by a roomtemperature water rinse.

[0068] The interstices 55 of the cut LSO and light guide arrays arepreferably filled with silicon dioxide powder of 1.53 micron particlesize to serve as reflector. It will be recognized that other reflectormaterials, such as titanium dioxide, aluminum oxide, magnesium oxide,barium sulfate, zinc oxide and Teflon powder, can also be utilized asreflectors. A small layer of Teflon powder is then used to ‘seal’ theSilicon dioxide reflector in the interstices of the arrays. This is toprevent permeation of bonding agents into the reflector from subsequentbonding processes. The LSO side of the LSO/light guide array is thenoptically bonded to the glass substrate of the NaI(Tl) array.

[0069] Two prototype detectors were fabricated using the methodsdescribed above. Testing of the prototype detectors required that theNaI(Tl) array be hermetically sealed. For the one prototype an aluminumhousing was used to hermetically seal the NaI(Tl) array only. Glass wasused as the hermetic seal for the second prototype, however, in thiscase the NaI(Tl), LSO and light guide were all enclosed.

[0070] Referring to FIGS. 3a-3 l, various configurations of detectorarrays are illustrated. It should be understood that the figures are notdrawn to scale. These figures present a perspective view of the types ofdetector blocks which could be incorporated in the detector head 15illustrated in FIG. 2. In the various embodiments illustrated herein,the scintillator can either be a single layered scintillator or can becomposed of two stacks of scintillator material of different decay timeseither using the same scintillator material in each layer or differentscintillator materials. Selection of the scintillator material for eachscintillation layer is application dependent. Table 1 herein providesexamples of various scintillator materials, but is not all inclusive.Other scintillator materials having similar properties or otherapplication dependent properties could also be substituted. Theembodiments can be further modified by varying the types of opticaldetectors which are used, i.e. photomultipliers and/or photodiodes. Anadditional variant is whether or not a segmented optical light guide isused. If a segmented optical light guide is used then the variant iswhether the configuration is inverted or non-inverted.

[0071] In this regard, in FIG. 3(a) one detector block 20 from thedetector head 15 illustrated in FIG. 2 incorporates discretescintillators 40 composed of two stacked different scintillatormaterials 140 and 240 of different decay times optically coupled to asegmented inverted light guide 50, which is preferably non-active, whichin turn is optically coupled to photomultiplier tubes (PMTs) 25. ThePMTs 25 are the optical detectors. This embodiment can be furthervaried, as shown in FIG. 3(b)with the addition a photodiode array 25′which is optically coupled to one of the scintillator arrays 20. ThePMTs 25 and photodiode arrays 25′ are the optical detectors.

[0072] Another embodiment is illustrated in FIG. 3(c). This embodimentincorporates discrete scintillators 40 composed of two stacked differentscintillator materials 140 and 240 of different decay times opticallycoupled to a segmented inverted light guide 50, which is preferablynon-active. The light guide 50 is optically coupled to a photodiodearray 25′ which serves as the optical detector. This embodiment can befurther modified, as shown in FIG. 3(d) which includes a secondphotodiode array 25′ optically coupled to one of the scintillatorarrays. The two photodiode arrays are the optical detectors. Asillustrated in FIG. 3(e), the embodiment illustrated in FIG. 3(c) can befurther modified by integrating the light guide function in the discretescintillators 40. In FIG. 3(f), the embodiment illustrated in FIG. 3(e)has been further modified by optically coupling a second photodiodearray 25′ to scintillator material 140.

[0073] FIGS. 3(g ₁) and (g ₂) yet another embodiment is illustrated.This embodiment incorporates discrete scintillators 40 composed of twostacked different scintillator materials 140 and 240 of different decaytimes optically coupled to a segmented non-inverted light guide 150,which is preferably non-active. The light guide 150 is optically coupledto PMTs 25.

[0074]FIG. 3(h) illustrates yet another embodiment of the design whichincorporates discrete scintillators 40 composed of two stacked differentscintillator materials 140 and 240 of different decay times opticallycoupled to a segmented non-inverted light guide 150, which is preferablynon-active. The light guide 150 is optically coupled to the PMTs 25 andin addition a photodiode array 25′ is optically coupled to one of thescintillator arrays 140.

[0075]FIG. 3(i) illustrates still another embodiment of the designincorporating discrete scintillators 40 composed of two stackeddifferent scintillator materials 140 and 240 of different decay timesoptically coupled to a segmented non-inverted light guide 150 which ispreferably non-active. The light guide 150 is optically coupled to aphotodiode array 25′. This embodiment can be further modified, byoptically bonding layer 140 to a second photodiode array 25′.

[0076]FIG. 3(k) is a perspective view of an alternative segmented lightguide 150′ from the detector block illustrated in FIG. 3(j) showing thatthe light depth and configuration of the segmentation is variabledepending upon the thickness of the light guide. FIG. 4 illustrates anembodiment of the design incorporating discrete scintillators 40composed of two stacked different scintillator materials 140 and 240 ofdifferent decay times optically coupled to a non-segmented light guide250, which is preferably non-active. The light guide 250 is opticallycoupled to the PMTs 25. With respect to each of these embodiments, andthe other embodiments described herein, selection of the scintillatormaterial for each scintillation layer is application dependent. Table 1herein provides examples of various scintillator materials, but is notall inclusive. Other scintillator materials having similar properties orother application dependent properties could also be substituted.

[0077] As illustrated in FIGS. 4, 6 and 8, in various embodiments, theplanar light guide 250 is continuous, i.e. non-segmented. Asillustrated, the continuous, planar light guide 250 has a first planarsurface that extends beyond the interfacing surface of the discretescintillator elements and has a second planar surface that extendsbeyond the interfacing surface of the optical detectors. An alternateembodiment is illustrated in FIGS. 4b and 6 b. In this alternateembodiment, the scintillators, which can either be a single layer 40 ofmaterial as illustrated in FIG. 6b, or two layers 240 and 140 asillustrated in FIG. 4b, are disposed between the optical detectors 25and a thin layer continuous, planar light guide 250′. In this regard,the planar light guide 250′, which can either be active or non-active,is disposed on the patient side of the scintillators. For use in PETapplications, the following High-Z scintillator materials are preferred:Cerium-doped Lutetium Oxyorthosilicate, (“LSO”), Cerium-doped LutetiumYttrium Oxyorthosilicate, (“LYSO”), Cerium-doped Lutetium GadoliniumOxyorthosilicate, (“LGSO”), Cerium-doped Gadolinium Oxyorthosilicate,(“GSO”), Cerium-doped Lutetium Aluminum Perovskite, (“LuAP”), andCerium-doped Yttrium Aluminum Perovskite, (“YAP”). Also, Cerium-dopedYttrium Oxyorthosilicate, (“YSO”), can be selected as a scintillatormaterial.

[0078] Referring to FIG. 5, the NaI(Tl) array 20′ was optically bondedto the segmented inverted light guide 50, which is preferablynon-active, for evaluation of the detector design. Testing of theNaI(Tl) array 20′ was conducted using standard nuclear spectroscopyinstrumentation. The array was evaluated for energy and ‘position’ spaceat 511 keV, 122 keV and 88 keV, using point sources. The ‘position’histograms 60, 62, and 64 for these three energies are provided in FIGS.9(a), 9(b) and 10, respectively, with Spectra Identification number27-18-1.spm. The position histogram as depicted exhibits the overlap ofall twelve rows. The twelve position peaks represent all 144 discretescintillator elements. Any mispositioning of the 144 peaks would resultin a degradation of the peak-to-valley. No such degradation isexhibited. The pulse height energy resolution measured at 88 keV, 122keV, and 511 keV are summarized as follows: Mean Pulse Height EnergyResolution per Energy crystal element  88 keV 9.42% 122 keV 9.45% 511keV 7.57%

[0079]FIG. 11 provides position information without the complexity ofinterpreting twelve overlapped rows. FIG. 11 shows the two dimensionalposition histogram 66 of the detector design exhibited in FIG. 5 wherethe scintillator layer is composed of NaI(Tl) layer segmented into the12×12 array. The light guide 50 is non-active, segmented and inverted.The detector is uniformly irradiated with 122 keV gamma-rays from Co-57.This data indicates that the NaI(Tl) array performance is comparable toexisting high resolution SPECT scanners while providing outstanding PETenergy, position and time resolution.

[0080] Evaluation of the bonded NaI(Tl)/LSO/light guide arrays (seehistograms 68 and 70 in FIGS. 12 and 13) requires consideration of thenaturally occurring 2.6% abundant Lu-176 element. The Lu- 176 isradioactive, producing energetic particles that interact in thescintillator to produce a background count rate of approximately 39counts per second per gram of LSO scintillator. Due to the high stoppingpower of LSO very few of the particles originating from the decay ofLu-176 actually escape from the LSO scintillator. Lu-176 backgroundevents do not present a problem in PET studies due to their uncorrelatednature, the system labels the events as randoms and are thus rejected.However, the Lu-176 background events which fall into the SPECT energywindow are a problem for they are counted as singles and are of the sameorder of magnitude as the signal rate in many Planar and SPECT studies.FIG. 14 exhibits two energy spectra, one for an LSO crystal irradiatedby Cs-137 72, the other spectra is for the Lu-176 background events 74.These spectra indicate standard energy discrimination techniques willnot successfully reject Lu-176 background events.

[0081] NaI(Tl) and LSO have scintillation decay times of 230 and 40nanosecs (ns) respectively. For SPECT studies, low energy photons arestopped in the NaI(Tl) crystal(s), producing scintillation events of 230ns decay time; whereas the Lu-1 76 background events of LSO are producedwith 40 ns decay. The electronics circuitry distinguishes betweenNaI(Tl) and LSO events based on the decay time signatures of the twoscintillators. The technique is known as pulse shape discrimination PSD.Approximately 99% of the Lu-176 background events must be rejected toprevent significant noise counts from occurring in the low count rateSPECT data.

[0082] The PSD technique utilized in the PET/SPECT detector involvesintegrating the detector charge at two points in time and then comparingthe ratio of the values. For LSO and NaI(Tl) separation, the firstsample is taken at 80 ns from the start of the integration and thesecond sample at 256 ns. The first sample is selected at 80 ns sincethis is where the maximum difference occurs for the LSO and NaI(Tl)scintillators. The LSO signal will be maximally above the NaI(Tl) signalat 80 ns for a given signal charge. The second sample at 256 ns is atthe end of the signal integration to provide a value to normalize outevent charge (energy). The two-sample shape discrimination circuit usedwith the PET/SPECT ASIC does not require additional shape discriminationcircuitry inside the ASIC. The external logic sequencing circuitrequests an intermediate and final energy sample from the externalenergy ADC. These two values can then be effectively ratioed anddiscriminated through a look-up logic memory device to determine if theevent came from the NaI(Tl) or LSO scintillator. The integrated signalsfor the NaI(Tl) and LSO is shown in FIG. 15. The deviation from a pureexponential behavior for LSO is caused by the filtering circuitrynecessary to cancel the light decay of NaI(Tl) for improved count rateperformance.

[0083] In order to utilize the two-sample shape discrimination, it isnecessary to ensure that the energy integrator is sufficiently linear atthe sample points for the NaI(Tl) and LSO scintillation detectorsignals. The integrated energy signal is sampled using an 8-bit flashconverter at approximately 80 ns (E1) and again at 256 ns (E2). Theenergies (E1) and (E2) are used to determine if the event occurred in aNaI(Tl) or an LSO crystal. The ratios of the energies (E1) and (E2) areused for the shape discrimination. A 65536×1 static RAM 75 can be usedto indicate the crystal type as shown in FIG. 16. The integrated X and Yvalues are digitized at 256 ns using flash converters with theintegrated energy signal as the reference to produce the (A+B)/Sum and(A+C)/Sum ratios. The X and Y ratios are used to determine the crystalin which the event occurred. A 65536×8 static RAM 77 can be used toindicate the crystal as shown in FIG. 17.

[0084]FIG. 18 exhibits the block diagram of the method used to evaluatethe prototype detectors for energy, position and time (see FIGS. 12 and13). The module 79 labeled “CTI Design Box” incorporates a preamp,timing filter amp, sum amp, CFD, and digital clock. The rest of theFigure illustrates standard nuclear spectroscopy instrumentation—such asis available from EG&G Ortec (e.g. 1995 EG&G Ortec catalog “ModularPulse-Processing Electronics and Semiconductor RadiationDetectors”)—processing for performing pulse shape discrimination. Forexample, those skilled in the art will appreciate that module 81 is astandard personal computer with analog to digital converters, module 83is for pulse shaping and includes a Constant Fraction Discriminator andthat module 85 is a Time-to-Amplitude Converter. Analysis of the twoprototype blocks in terms of energy and crystal identificationdemonstrates that when the devices are operated in the SPECT mode allthe individual NaI(Tl) discrete element detectors can be identified at140 keV. Further, the average pulse height energy resolution at 140 keVis 10.3% for prototype detector block #1 and 10.0% for block #2. Themedian pulse height energy resolution is below 10.0% in both cases. Thisperformance is comparable to current SPECT imaging systems. Whenoperated in the PET mode, all the individual NaI(Tl) and LSO discreteelement detectors are easily identified. The average pulse height energyresolution measured at 511 keV is below 10.1% for both scintillatorlayers (less than 8.0% for the NaI(Tl) layer). This indicates excellentPET performance comparable to existing PET imaging systems.

[0085]FIG. 19 exhibits the block diagram used in the determination ofcoincidence performance. Similarly FIG. 19 illustrates standard nuclearspectroscopy instrumentation. For example module 87 is a fast shapingamplifier along with a constant fraction discriminator, and module 93 isa time-to-amplitude converter with a single channel analyzer. The outputat module 97 is illustrated in FIG. 21. FIG. 20 exhibits the geometricconfiguration used in measuring the coincidence timing. The figureillustrates 2×2 center crystals for each scintillator layer of the onecrystal block is used in coincidence with the 144 crystal elements ofeach layer of the opposing block. Time spectra are then measured foreach of the 144 crystal elements of each scintillator layer. FIG. 21displays the coincidence time spectra for each layer combination. Thetime centroids are symmetric, but the centroid position is dependentupon the location of the discrete crystal element with respect to thePMT, and reflects the spatial uniformity in the anode output (see FIGS.22 & 23). The time centroid shifts can be corrected via a lookup table.Time resolution is scintillator layer combination dependent. The LSO-LSOcombination exhibits 1.6 nsec time resolution, indicating PETcoincidence timing with 6 nsec time window is feasible.

[0086] The line spread function (LSF) method was used to assess thespatial resolution of the prototype detector blocks. FIG. 24 exhibitsthe geometric arrangement used in the LSF measurement. FIG. 25 exhibitsthe step wise data of the LSF for the layer combinations, and Table 2provides the LSF data in terms of full width at half maximum (FWHM) ofthe LSF. TABLE 2 Line spread functions for the layer combinations of thedetector design exhibited in FIG. 3(a) where scintillator layer (1) iscomposed of Nal [TI] and scintillator layer (2) is composed of LSO. Thedetector is irradiated with 511 keV gamma rays from an F-18 line source.See FIG. (24) for geometric arrangement. Combination FWHM [mm] StdDev[mm] LSO-LSO (back-back) 3.5 0.2 Nal-LSO (front-back) 3.0 0.4 LSO-Nal(front-back) 3.1 0.3 Nal-Nal(front-front) 2.8 0.5

[0087] The LSF data indicates the depth of interaction (DOI) effects andthat a reconstructed spatial resolution of less than 4 mm is possible.

[0088]FIG.26. displays the general system architecture for applicationin PET/SPECT medical imaging.

Absolute Sensitivity of PET/SPECT

[0089] The following calculation was done to predict the absolutesensitivity of the proposed PET/SPECT medical imaging system. Absolutesensitivity is defined as the ratio of the events detected by the systemto those emitted from a line source placed at the center of thetomograph. The length of the source is the same as the axial length ofthe scanner. If the sensitivity of the detector to single events isknown then the calculation of absolute sensitivity is the product of thefraction of solid angle and the square of the singles sensitivity.

η_(Ab)=η_(Geom)·η_(Det) ²

[0090] For the NaI/LSO detector, both a Monte Carlo simulation and ameasurement were done. These are summarized in the table below:Coincidence Sensitivity of NaI/LSO Detector Calculated CalculatedMeasured Absolute Relative Relative LSO/LSO 12.7% 52.7% 50.1% LSO/NAI 4.8% 19.9% 20.4% NaI/LSO  4.8% 19.9% 20.4% NaI/NaI  1.8%  7.5%  9.0%

[0091] Assuming two 50 cm transaxial by 40 cm axial detectors, directlyopposing and 72 cm face to face, the fraction of the solid angle isfound to be 6.2%. Using the total detector sensitivity from table above,the absolute sensitivity is found to be 1.5%.

[0092] For whole body scanning, the body is longer than the axial extentof the scanner. Then, if one arbitrarily picks a length for the sourceof 70 cm, it is possible to put the absolute sensitivity of the scannerin perspective by comparing to existing scanners. The table belowcompares the PET/SPECT, Siemens ECAT ART, and Siemens ECAT HR+ for botha line source of axial extent and one of 70 cm. Absolute SensitivityComparison of PET/SPECT 70 cm Axial Length Source Source MeasuredCalculated Calculated ART 1.04% 1.10% 0.25% HR+ 2.94% 2.80% 0.62%PET/SPECT 1.50% 0.86%

[0093] Count-Rate Performance of PET/SPECT

[0094] To assess the performance of the PET/SPECT in an imagingsituation, the proposed machine was compared to an existing Siemens ECATART tomograph. Since the count-rate performance of the machine is afunction of the detector singles rate, this needs to be known along withthe trues sensitivity. The randoms rate can be computed from the singlesrates and the coincidence window.

[0095] A typical whole-body study performed on an ART was used as abench mark. The study consisted of five bed steps ranging from thepatients nose to just above the bladder. The variation in the count-ratewith respect to bed step was minimal. Due to this, the rates wereaveraged to give the following results: ART Whole-body Study 5 mCiInjection Trues 20666 cps Randoms 10166 cps Singles/Block 27716 cps

[0096] The singles sensitivity of the ART blocks and dead-time areobtained by fitting the singles rates obtained in a count-rate studydone with a 20 cm uniform phantom filled with ¹⁸F. To model the responseof the system to the subject, the trues were set so that for the singlesrate of the subject study, the trues also matched. The randoms werecalculated from the singles and the coincidence window. The dead-time isbased on the fitted dead-time function. This process lets theperformance of the system be modeled with respect to activity. The NECrate shown in the right most column did not include a scatter term andwas simply T²/(T+2*R).

[0097] For PET/SPECT, results were used from the Monte Carlo model ofabsolute sensitivity to obtain ratios for the singles and truessensitivity. These values are given in the table below: PET/SPECTSensitivity (cps/uCi/cc) Trues Sensitivity 816000Singles/Block/Sensitivity 175000

[0098] The dead-time of the system is modeled based on the electronicdead-time of 320 nS and the following relation:$S_{Observed} = {S_{Incident} \cdot \frac{\exp \left( {{- 8} \cdot S_{Incident} \cdot \tau_{dead}} \right)}{\left( {1 + {S_{Incidnent} \cdot \tau_{dead}}} \right)}}$

[0099] This relation takes into account the overlapped detectorstructure that forces eight surrounding detectors to be dead during theprocessing of an event in one detector.

[0100] With these values, it is possible to predict the performance ofthe system to different injection levels as shown in the followingchart: Whole-body Imaging Performance of PET/SPECT PET/SPECT 40 cm fov,ART 320 ns Deadtime Singles Trues Randoms NEC Singles Trues Randoms NECActivity¹ cps Livetime cps cps cps cps Livetime cps cps cps 0.500 8928636.3% 41550 87111 8001 37304 18.2% 74157.6 53437 30378 0.475 86210 38.0%41306 82270 8289 36975 19.8% 76690.0 52499 32371 0.450 83031 39.8% 4096777300 8582 36548 21.5% 79092.1 51294 34432 0.425 79742 41.6% 40524 722158879 36015 23.4% 81320.9 49809 36549 0.400 76338 43.6% 39965 67030 917835368 25.5% 83326.4 48035 38704 0.375 72814 45.7% 39279 61762 9477 3459827.8% 85051.1 45965 40873 0.350 69163 48.0% 38452 56431 9771 33694 30.3%86429.1 43596 43025 0.325 65379 50.3% 37470 51063 10058 32648 33.0%87384.9 40930 45119 0.300 61455 52.9% 36317 45685 10330 31447 35.9%87831.9 37974 47102 0.275 57383 55.5% 34975 40330 10579 30081 39.1%87671.7 34746 48906 0.250 53155 58.4% 33424 35038 10794 28537 42.5%86792.2 31271 50443 0.225 48762 61.4% 31642 29852 10961 26802 46.3%85065.7 27584 51601 0.200 44195 64.6% 29604 24826 11058 24862 50.5%82347.7 23736 52235 0.175 39445 68.1% 27283 20020 11056 22703 55.0%78474.3 19792 52163 0.150 34499 71.8% 24647 15502 10916 20308 59.9%73260.0 15837 51147 0.125 29347 75.7% 21664 11355 10577 17662 65.2%66495.2 11979 48883 0.100 23975 79.9% 18294 7671 9950 14747 71.0%57943.4 8351 44979 0.075 18370 84.4% 14494 4558 8898 11543 77.3% 47337.85117 38923 0.050 12517 89.2% 10216 2142 7198 8032 84.3% 34377.8 247730047 0.025 6400 94.4% 5405 567 4468 4192 91.8% 18725.3 675 17467 0.0000 100.0% 0 0 0 0 100.0% 0 0 0

[0101] From the foregoing description, it will be recognized by thoseskilled in the art that a detector array, having particular applicationin Single Photon Imaging which includes traditional Gamma Cameras,Planar Imaging, Single Photon Emission Computed Tomography (SPECT) withor without Coincidence Photon Imaging and Positron Emission Tomography(PET), offering advantages over the prior art has been described andshown. Specifically, the detector array of the present inventionincorporates either a single scintillator layer or two, stacked discretescintillator layers that can be the same scintillator material or of twodifferent scintillator materials. In either case the different layersare composed of materials that have distinctly different decay times.The variants in these figures are the types of optical detectors whichare used, i.e. photomultipliers and/or photodiodes. Additionally, theoptical light guide can be integral with the scintillators or opticallybonded thereto. Further, the planar light guide can be active ornon-active. In either of these variants, the planar light guide can besegmented or non-segmented. And, if segmented, can either be inverted ornon-inverted.

[0102] While a preferred embodiment has been shown and described, itwill be understood that it is not intended to limit the disclosure, butrather it is intended to cover all modifications and alternate methodsfalling within the spirit and the scope of the invention as defined inthe appended claims.

Having thus described the aforementioned invention, we claim:
 1. Ascintillation detector array for encoding energy, position and timecoordinates of gamma ray interactions for use in Positron EmissionTomography imaging, said scintillation detector array comprising: aplurality of discrete scintillator elements which interact with incidentgamma-rays to produce a quantifiable number of scintillation photons,wherein each of said plurality of discrete scintillators is composed ofa first layer having a first selected decay time and a second layerhaving a second selected decay time, wherein said first selected decaytime is not equal to said second selected decay time, and furtherwherein said first layer is composed of a first selected scintillatormaterial and said second layer is composed of a second selectedscintillator material and wherein said first and second selectedscintillator materials are stacked one upon the other, whereby a pulseshape discrimination technique is used to determine which said layer thegamma ray interacts; an optical detector associated with each of saidplurality of discrete scintillator elements and positioned for sensingand quantifying said scintillation photons exiting each of saidplurality of discrete scintillator elements; a continuous light guidehaving first and second planar surfaces disposed between said pluralityof discrete scintillator elements and said associated optical detectorsfor distributing scintillation photons exiting said plurality ofdiscrete scintillators to said associated optical detectors; and a meansoperatively associated with said scintillation detector array fordetermining time, energy, depth and transverse and longitudinal positioncoordinates of gamma ray interactions in said plurality of discretescintillator elements.
 2. The scintillator detector array of claim 1wherein said first and said second layers are composed of High-Zscintillator materials.
 3. The scintillation detector array of claim 1wherein said plurality of discrete scintillator elements, which interactwith incident gamma-rays to produce a quantifiable number ofscintillation photons, is arranged in an (m)×(n) array, and saidplurality of optical detectors is arranged in an (q)×(p) array, whereinsaid plurality of optical detectors is for sensing and quantifying saidscintillation photons exiting each of said plurality of discretescintillator elements.
 4. The scintillator detector array of claim 3wherein said (m)×(n) array equals said (q)×(p) array.
 5. Thescintillator detector array of claim 3 wherein said (m)×(n) array doesnot equal said (q)×(p) array.
 6. The scintillator detector array ofclaim 2 wherein said first and said second layer of each of saidplurality of discrete scintillator elements is composed of LSO.
 7. Thescintillator detector array of claim 2 wherein said High-Z scintillatormaterial is selected from a group consisting of LSO, LYSO, LGSO, GSO,LuAP, and YAP.
 8. The scintillator detector array of claim 2 whereinsaid first layer is composed of a first selected scintillator materialand said second layer is composed of a second selected scintillatormaterial.
 9. The scintillator detector array of claim 8 wherein saidfirst selected scintillator material and said second selectedscintillator material are selected for use in techniques for separatinglow and high energies.
 10. The scintillator detector array of claim 8wherein said first selected scintillator material and said secondselected scintillator material are selected for use in techniques fordetermining depth of interaction of the gamma rays with said pluralityof discrete scintillator elements.
 11. The scintillator detector arrayof claim 8 wherein said first selected scintillator material and saidsecond selected scintillator material are selected for use in techniquesfor distinguishing pulse heights of gamma ray interactions.
 12. Thescintillator detector array of claim 1 wherein said first selectedscintillator material is YSO and said second selected scintillatormaterial is a High-Z scintillator material.
 13. The scintillatordetector array of claim 1 wherein said first selected scintillatormaterial is LSO and said second selected scintillator material is GSO.14. The scintillator detector array of claim 1 wherein said firstselected scintillator material is YSO and said second selectedscintillation material is LSO.
 15. The scintillator detector array ofclaim 1 wherein said light guide is active.
 16. The scintillationdetector array of claim 1 wherein said light guide is non-active.
 17. Ascintillation detector array for encoding energy, position and timecoordinates of gamma ray interactions for use in Positron EmissionTomography imaging, said scintillation detector array comprising: aplurality of discrete scintillator elements which interact with incidentgamma-rays to produce a quantifiable number of scintillation photons,wherein each of said plurality of discrete scintillators is composed ofa first layer having a first selected decay time and a second layerhaving a second selected decay time, wherein said first selected decaytime is not equal to said second selected decay time, and furtherwherein said first and said second layers are composed of High-Zscintillator materials, and further wherein said first layer is composedof a first selected scintillator material and said second layer iscomposed of a second selected scintillator material and wherein saidfirst and second selected scintillator materials are stacked one uponthe other, whereby a pulse shape discrimination technique is used todetermine which said layer the gamma ray interacts; an optical detectorassociated with each of said plurality of discrete scintillator elementsand positioned for sensing and quantifying said scintillation photonsexiting each of said plurality of discrete scintillator elements; acontinuous light guide having first and second planar surfaces disposedbetween said plurality of discrete scintillator elements and saidassociated optical detectors for distributing scintillation photonsexiting said plurality of discrete scintillators to said associatedoptical detectors; and a means operatively associated with saidscintillation detector array for determining time, energy, depth andtransverse and longitudinal position coordinates of gamma rayinteractions in said plurality of discrete scintillator elements. 18.The scintillation detector array of claim 17 wherein said plurality ofdiscrete scintillator elements, which interact with incident gamma-raysto produce a quantifiable number of scintillation photons, is arrangedin an (m)×(n) array, and said plurality of optical detectors is arrangedin an (q)×(p) array, wherein said plurality of optical detectors is forsensing and quantifying said scintillation photons exiting each of saidplurality of discrete scintillator elements.
 19. The scintillatordetector array of claim 18 wherein said (m)×(n) array equals said(q)×(p) array.
 20. The scintillator detector array of claim 18 whereinsaid (m)×(n) array does not equal said (q)×(p) array.
 21. Thescintillator detector array of claim 17 wherein said light guide isactive.
 22. The scintillation detector array of claim 17 wherein saidlight guide is non-active.
 23. A scintillation detector array forencoding energy, position and time coordinates of gamma ray interactionsfor use in Positron Emission Tomography imaging, said scintillationdetector array comprising: a plurality of discrete scintillator elementswhich interact with incident gamma-rays to produce a quantifiable numberof scintillation photons, wherein each of said plurality of discretescintillators is composed of a first layer having a first selected decaytime and a second layer having a second selected decay time, whereinsaid first selected decay time is not equal to said second selecteddecay time, and further wherein said first and said second layers arecomposed of High-Z scintillator materials, and further wherein saidfirst layer is composed of a first selected scintillator material andsaid second layer is composed of a second selected scintillator materialand wherein said first and second selected scintillator materials arestacked one upon the other, whereby a pulse shape discriminationtechnique is used to determine which said layer the gamma ray interacts;an optical detector associated with each of said plurality of discretescintillator elements and positioned for sensing and quantifying saidscintillation photons exiting each of said plurality of discretescintillator elements; a continuous light guide having first and secondplanar surfaces optically bonded to said plurality of discretescintillator elements, whereby said plurality of discrete scintillatorelements is disposed between said light guide and said opticaldetectors, wherein said plurality of discrete scintillator elementsdistribute scintillation photons exiting said plurality of discretescintillators to said associated optical detectors; and a meansoperatively associated with said scintillation detector array fordetermining time, energy, depth and transverse and longitudinal positioncoordinates of gamma ray interactions in said plurality of discretescintillator elements.
 24. The scintillation detector array of claim 23wherein said plurality of discrete scintillator elements, which interactwith incident gamma-rays to produce a quantifiable number ofscintillation photons, is arranged in an (m)×(n) array, and saidplurality of optical detectors is arranged in an (q)×(p) array, whereinsaid plurality of optical detectors is for sensing and quantifying saidscintillation photons exiting each of said plurality of discretescintillator elements.
 25. The scintillator detector array of claim 24wherein said (m)×(n) array equals said (q)×(p) array.
 26. Thescintillator detector array of claim 24 wherein said (m)×(n) array doesnot equal said (q)×(p) array.
 27. The scintillator detector array ofclaim 23 wherein said first and said second layer of each of saidplurality of discrete scintillator elements is composed of LSO.
 28. Thescintillator detector array of claim 23 wherein said High-Z scintillatormaterial is selected from a group consisting of LSO, LYSO, LGSO, GSO,LuAP, and YAP.
 29. The scintillator detector array of claim 23 whereinsaid first layer is composed of a first selected scintillator materialand said second layer is composed of a second selected scintillatormaterial.
 30. The scintillator detector array of claim 29 wherein saidfirst selected scintillator material and said second selectedscintillator material are selected for use in techniques for separatinglow and high energies.
 31. The scintillator detector array of claim 29wherein said first selected scintillator material and said secondselected scintillator material are selected for use in techniques fordetermining depth of interaction of the gamma rays with said pluralityof discrete scintillator elements.
 32. The scintillator detector arrayof claim 29 wherein said first selected scintillator material and saidsecond selected scintillator material are selected for use in techniquesfor distinguishing pulse heights of gamma ray interactions.
 33. Thescintillator detector array of claim 29 wherein said first selectedscintillator material is YSO and said second selected scintillatormaterial is a High-Z scintillator material.
 34. The scintillatordetector array of claim 29 wherein said first selected scintillatormaterial is LSO and said second selected scintillator material is GSO.35. The scintillator detector array of claim 29 wherein said firstselected scintillator material is YSO and said second selectedscintillation material is LSO.
 36. The scintillator detector array ofclaim 23 wherein said light guide is active.
 37. The scintillationdetector array of claim 23 wherein said light guide is non-active.
 38. Ascintillation detector array for encoding energy, position and timecoordinates of gamma ray interactions for use in Positron EmissionTomography imaging, said scintillation detector array comprising: aplurality of discrete scintillator elements which interact with incidentgamma rays to produce a quantifiable number of scintillation photons,wherein each of said plurality of discrete scintillators is composed ofa first layer having a first selected decay time and a second layerhaving a second selected decay time, wherein said first selected decaytime is not equal to said second selected decay time, and furtherwherein said first layer is composed of a first selected scintillatormaterial and said second layer is composed of a second selectedscintillator material and wherein said first and second selectedscintillator materials are stacked one upon the other, whereby a pulseshape discrimination technique is used to determine which said layer thegamma ray interacts; an optical detector associated with each of saidplurality of discrete scintillator elements and positioned for sensingand quantifying said scintillation photons exiting each of saidplurality of discrete scintillator elements wherein said plurality ofdiscrete scintillator elements, which interact with incident gamma raysto produce a quantifiable number of scintillation photons, is arrangedin an (m)×(n) array, and said plurality of optical detectors is arrangedin an (q)×(p) array, wherein said (m)×(n) array does not equal said(q)×(p) array and further wherein said plurality of optical detectors isfor sensing and quantifying said scintillation photons exiting each ofsaid plurality of discrete scintillator elements; a continuous lightguide having first and second planar surfaces disposed between saidplurality of discrete scintillator elements and said associated opticaldetectors for distributing scintillation photons exiting said pluralityof discrete scintillators to said associated optical detectors; and ameans operatively associated with said scintillation detector array fordetermining time, energy, depth and transverse and longitudinal positioncoordinates of gamma ray interactions in said plurality of discretescintillator elements.
 39. The scintillator detector array of claim 38wherein said first and said second layers are composed of High Zscintillator materials.
 40. The scintillator detector array of claim 39wherein said first and said second layer of each of said plurality ofdiscrete scintillator elements is composed of LSO.
 41. The scintillatordetector array of claim 39 wherein said High-Z scintillator material isselected from a group consisting of LSO, LYSO, LGSO, GSO, LuAP, and YAP.42. The scintillator detector array of claim 39 wherein said first layeris composed of a first selected scintillator material and said secondlayer is composed of a second selected scintillator material.
 43. Thescintillator detector array of claim 42 wherein said first selectedscintillator material and said second selected scintillator material areselected for use in techniques for separating low and high energies. 44.The scintillator detector array of claim 42 wherein said first selectedscintillator material and said second selected scintillator material areselected for use in techniques for determining depth of interaction ofthe gamma rays with said plurality of discrete scintillator elements.45. The scintillator detector array of claim 42 wherein said firstselected scintillator material and said second selected scintillatormaterial are selected for use in techniques for distinguishing pulseheights of gamma ray interactions.
 46. The scintillator detector arrayof claim 38 wherein said first selected scintillator material is YSO andsaid second selected scintillator material is a High Z scintillatormaterial.
 47. The scintillator detector array of claim 38 wherein saidfirst selected scintillator material is LSO and said second selectedscintillator material is GSO.
 48. The scintillator detector array ofclaim 38 wherein said first selected scintillator material is YSO andsaid second selected scintillation material is LSO.
 49. The scintillatordetector array of claim 38 wherein said light guide is active.
 50. Thescintillation detector array of claim 38 wherein said light guide isnon-active.